A relief is a three-dimensional (3D) sculpture or sculpted surface having a relatively shallow depth with respect to a background plane. For centuries, sculptors have created reliefs by imagining a compressed-depth image of a 3D object and sculpting the image features into the surface of a material, such as a rectangular block of stone, at their compressed depths. An example of a relief having an extremely shallow depth (sometimes referred to as a bas-relief) is the sculpted bust of a person or other subject on the face of a coin. In the example of a coin, the relief conveys to the observer an image of the person or other subject. The manner in which light falls upon a relief contributes to the 3D effect. In the example of a coin, illumination is typically very even, i.e., non-directional, across the relief. As a result of this type of uniform illumination and the shallow depth of the relief, the image that the relief conveys appears very uniform to an observer over a wide range of viewing angles. In an example in which the subject of the relief is a person's face, the face appears essentially the same to the observer regardless of the viewing direction. Most reliefs are intended by the sculptor to be viewed essentially head-on, i.e., from a direction along a normal to the background plane. A deep or unevenly illuminated relief will not appear quite the same when viewed from different directions. Though a sculptor may not think in such terms, the sculptor has in mind a certain fixed association between light reflected from the relief surface and the surface normal.
Computer-assisted techniques for producing reliefs from 3D digital models have been described. The techniques address the above-referenced depth compression and other issues. The output of such techniques is a digital model of the relief. Such a digital model could then be used to drive an automated fabrication machine that sculpts the relief from a block of material or to produce a mold for manufacturing the relief through a molding process.